The present invention relates to a process for preparing printing forms or metallized images by electrophotographic means through electrostatic chargeup, image-wise exposure and tonering of a photoconductor layer applied to an electroconductive base material; fixing the tonered image areas; removing the photoconductor layer in the nonimage areas by decoating and, where appropriate, etching the bared base material surface.
Processes for preparing printing forms, such as offset printing plates, and printed circuits by electrophotographic means are known. For instance, offset printing plates with a photoconductor layer which contains zinc oxide can be prepared by subjecting the plate, which is imagewise covered with toner after development, to controlled hydrophilization with a hydrophilizing agent, for example, by treatment with ferricyanide solution, in the nonimage areas. As a result, differentiation is obtained between ink-accepting, hydrophobic, tonered image areas and the hydrophilized layer surface. The disadvantage of such a process is the necessity of having to repeat the hydrophilizing step more than once, and the relatively short print run.
To overcome the disadvantages described, there has been a trend, in the field of electrophotographically presensitized offset printing plates, toward systems where the hydrophilizing step is replaced by a decorating step. It is thus possible, according to German Auslegeschrift No. 1,117,391 (corresponding to British Pat. No. 944,126), to obtain image differentiation by using as the binder of the photoconductive layer a high molecular weight compound that contains alkali-solubilizing groups. Tonering and fixing the tonered image areas yields a mask which restricts decorating with an alkaline decoater to the nonimage areas of the photoconductor layer. By using a hydrophilic base material, such as an electrochemically or mechanically roughened and anodized aluminum base, it is thus possible to obtain differentiation between hydrophobic toner image or hydrophobic photoconductor layer, on the one hand, and the bared hydrophilic base material surface, on the other hand.
For decorating, solutions of inorganic and/or organic alkaline substances in water and/or organic solvents are used. Sparingly volatile amines or amino alcohols, which are used in concentrations between about 1 and 5% as solution in polyhydric alcohols and/or water, are considered particularly suitable. Application can be effected not only by simply wiping, for example, with an impregnated pad of absorbent cotton, or by dipping into a decorating bath, but also by means of appropriate mechanized apparatus in which the alkaline liquid is applied.
It is known (German Auslegeschrift No. 1,117,391 and German Pat. No. 2,322,046, corresponding to British Pat. No. 1,465,926) that good decoating action is obtained in particular when the decoater contains solvents, especially alcohols. Such solvents are capable of dissolving the organic photoconductor which is dissolved in the binder matrix of the mentioned layers, thereby speeding up the decoating step. This is of particular importance for mechanized processing, where the required decorating rate and service life of decorator are high. Solvent-containing decoaters also offer advantages with regard to machine soiling, since, by virtue of their dissolving power vis-a-vis the photoconductor, they prevent undesirable photoconductor deposits in the apparatus. Solvent-free decoater compositions cannot be used for this purpose.
On the other hand, solvent-containing decoaters have a number of disadvantages. For instance, when amines are used it is generally not possible to avoid a certain odor nuisance. For safety reasons, furthermore, the choice of solvents which can be used is restricted with respect to flash point or, alternatively, it is necessary to take expensive safety precautions. Finally, the toner of the developer--in general, polymers without alkali-solubilizing groups--frequently exhibits a certain solubility or swellability in the solvents used. This is true in particular of liquid developers of the dispersimer type as known, for example, from German Auslegeschrift No. 2,114,773, corresponding to U.S. Pat. No. 3,753,760. Solubility or swellability of the fixed toner during the decoating step results in considerable losses, particularly in the case of fine image elements. The decoater, passing through the toner mask, also attacks the photoconductor layer underneath; this can give rise to etching underneath the mask and to the subsequent float-off of the toner mask. This is also true of toner masks from dry developers, although--owing to the different manufacturing process--to a reduced degree.
The switch in preparing printing forms to photoconductive layers that contain small amounts of photoconductor dissolved in the binder and, in addition, very high amounts of photoconductive organic pigments, as described in German Offenlegungsschrift 34 17 951 leads (in the case of using solvent-containing photoconductor layers) to undesirable, significantly slower decoating speeds which are no longer sufficient for practical use. Complete decoating cannot be guaranteed in these cases.